Twins sometimes use eachother as pillows. When they both have the same DNA (because they started out as one baby and turned into two) it tries to make them into one body again because that’s what DNA is for.

When the twins don’t have the same DNA (because they started out as two babies) their bodies will just think the other baby is a pillow. This is unhealthy and sometimes hinders growth. In conclusion human twins are not a very good idea.

@Zyx Yes, I am serious, you did not get that from a science class I am sure. The other baby is not a pillow in the case of twins. Sometimes a twin does not develop correctly, or gets squeezed so to speak by one that is stronger. Especially in the cas of faternal twins it seems likely one twin may not prosper since miscarriage is so high to begin with even with singletons. Estimated one in three pregnancies do not go to term, one in five known pregnancies. Part of that can be something wrong with the mother, and part a genetic problem with the embryo. So, every embryo has probably around a 20% risk of not making it anyway.

Maybe you are confusing the occurance of people having what is thought to be tumors, and then finding out it is a twin that had not matured, that does happen.

The twins the OP is talking about are identical twins that develop together, at the same rate as one entity. Could be practically one body and two heads, but the torsoe is not exactly like a normal sungle body typically, but they may share vital organs like a heart, kidney, which makes it obviously very difficult to seperate them. It all depends where the two are attached whether they can succesfully be separated after birth, and sometimes there is a weaker twin even in that scenerio.

@Zyx – You might be thinking of cases in which there are two fetuses, but one fails to develop early on and is “resorbed” by the surviving fetus. It does happen but is rare. You might want to check out the Wikipedia article on parasitic twins and the one on twins in general; there’s some interesting stuff there.